If all Delivery Controllers in a XenDesktop site fail, you can configure the Virtual Delivery Agent (VDA) (for Server OS machines and Desktop OS machines) to operate in high availability mode so that users can continue to access and use their desktops. In high availability mode, the VDA will accept direct ICA connections from users, rather than connections brokered by the controller.
If communication with the Controller fails, high availability mode is initiated only after a set period of time has elapsed. By default, this is 300 seconds (5 minutes) but you can configure the time period.
Once in high availability mode (which is enabled for 30 days), the VDA attempts to register with a Controller for up to 30 days, while the user continues to use the desktop in this mode. When the Controller later becomes available, the desktop registers and the user's session continues uninterrupted, but any subsequent connection is brokered by the Controller as normal. If after 30 days the desktop is unable to register with the Controller, the desktop stops listening for connections and is no longer available. This means the administrator has 30 days in which to repair the Controller infrastructure and should not become reliant upon high availability mode.
High availability mode is suitable only for use with dedicated desktops, where the mapping between the user and the DA is known. You cannot configure high availability mode for use with pooled desktops.
To enable high availability mode, you:
Set the registry keys
To configure the VDA so that it operates in high availability mode when necessary, add the following registry key(s). You must do this after the VDA has been installed.
1 = enables high availability mode
0 (zero) = disables high availability mode
Value: number of seconds. The default is 300 seconds.
Prepare an ICA launch file
To establish a direct ICA connection to desktops, provide users with an ICA launch file that they can use should communication with the Controller fail. You must create an ICA launch file for each user who requires this feature; Citrix does not create or distribute ICA files for this purpose. For information on how to create ICA files, see http://support.citrix.com/article/CTX127392.
You will need to tell users when it is appropriate to use this ICA launch file and where they can access it from.
High availability mode limitations
High availability mode is suitable for use only with dedicated desktops; you cannot configure this for use with pooled desktops.
In high availability mode, some features are unavailable. These include:
High availability mode persists for up to 30 days only, after which the desktop is no longer available.